perf: Fix a build error with some GCC versions

Fix this:

 util/cgroup.c: In function ‘open_cgroup’:
 util/cgroup.c:16:16: error: ‘saved_ptr’ may be used uninitialized in this function
 util/cgroup.c:16:16: note: ‘saved_ptr’ was declared here

Apparently newer GCC (4.6) can figure out that this variable is properly
initialized - but some versions of GCC (such as 4.5.2) need help.

Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Cc: Arnaldo Carvalho de Melo <acme@infradead.org>
LKML-Reference: <new-submission>
Signed-off-by: Ingo Molnar <mingo@elte.hu>